Writing unit tests: It´s now or never!

Unit testing is an important quality assurance method but a challenge in the context of legacy code. We share our experiences in trying to create tests for existing code by means of two examples from a large industrial software system and list common testability issues observed. Although much effort and resources have been invested, the retrofitting of unit tests was only partially successful. Our findings are that the probability that code without initial tests will ever get covered by unit tests is very low. Only the existence of unit tests ensures testability of the code under test. Besides finding defects unit tests also ensure modular design, which is the base for reuse, maintainability and testability. As a consequence, code and tests should be written together, ideally in a test-driven way.